1. Field of the Invention
The invention relates to a millimeter-wave balanced mixer, and in particular to a quasi-optical mixer fabricated in planar form, i.e. fabricated as a microwave integrated circuit.
2. Description of the Prior Art
The technology related to microwave integrated circuit planar balanced mixers has progressed significantly in recent years. Mixers have been fabricated which are capable of operating at ever increasing RF frequency. One such mixer circuit is shown in U.S. Pat. No. 4,215,313, issued July 29, 1980 to Chang et al. and assigned to the assignee of the present invention. That patent, entitled "Dielectric Image Guide Integrated Harmonic Pumped Mixer" shows a mixer circuit employing a reverse poled diode pair (16, 18) mounted within a mixer cavity. LO (local oscillator) power is provided to the mixer cavity through a first dielectric image guide strip and RF (radio frequency) energy is provided to the mixer cavity through a second dielectric image guide strip. The cross-sectional dimensions of each guide strip are determined by the frequency of the signal to be propagated through the strip. For a given frequency, the waveguide height (narrower dimension) known as the critical height, may be determined for which the waveguide will support only the fundamental waveguide mode E.sup.y.sub.11. The higher the frequency of the wave being propagated, the smaller is the dimension. For high frequencies, i.e., 100 to 300 GHz and above, the dimensions are so small and tolerances so critical, that satisfactory waveguides are difficult to fabricate.
A further illustration of the state of the art of microwave integrated circuit planar high frequency mixers is the U.S. Pat. No. 4,276,655 issued June 30, 1981 to Kraemer et al. That patent, titled "Integrated Circuit Planar High Frequency Mixer" shows a truly planar crossbar signal mixer having both LO power and RF power coupled by waveguides to the same side of the substrate on which is formed the integrated circuit mixer. The circuit includes the mixer diodes, a crossbar, and a planar microwave integrated circuit low pass IF (intermediate frequency) output filter. An alternate embodiment shows the LO power coupled to one side (major surface) of the substrate and RF energy coupled via the other side (major surface) of the substrate. The device was operated at a design frequency centered at 11.8 GHz.
A copy of both the above discussed patents accompanies this application for patent and each provides useful background information and for this reason is incorporated herein by this reference.
An explanation of what is meant by "quasi-optical" with reference to high frequency mixers may be found in "A Simple Quasi-Optical Mixer for 100-120 GHz" by A. R. Kerr et al appearing in the IEEE Microwave Theory and Techniques International Microwave Symposium Digest, June 1977, a copy of which accompanies this application for patent.